1. Field of the invention
The present invention is generally concerned with beveling or grooving an ophthalmic lens.
2. Description of the prior art
As is known, when the ring or surround of the eyeglass frame in which an ophthalmic lens is to be mounted comprises an annular groove, commonly called a bezel, for retaining the lens, it is necessary to form on the peripheral edge surface of the ophthalmic lens, after trimming the latter to the contour of said ring or surround, a rib or bevel, generally of triangular transverse cross-section, adapted for its engagement in said groove in the latter.
Likewise, it is necessary to form a groove in it when the ring or surround of the eyeglass frame concerned comprises a tang and/or filament for retaining the lens.
In the following description and for reasons of convenience only, reference will more often than not be made only to the beveling needed to form a rib or bevel, but it is to be understood that the operations concerned could equally well relate to the grooving necessary to form a groove.
In practice, this beveling and the trimming which preceeds it are carried out on a machine, usually called a grinding machine, comprising at a machining station and on a frame a first support shaft which carries at least one beveling or grooving grinding tool and which is mounted so as to rotate when driven by a drive motor, and a second support shaft disposed parallel to the first and likewise rotatable, adapted to grip axially an ophthalmic lens in line with said beveling or grooving griding tool so that the edge of said ophthalmic lens is in contact with the latter.
It is, of course, important that the bevel formed on an ophthalmic lens processed by means of a grinding machine of this kind is actually on the edge surface thereof, between its circumferential edges.
In practice, in order to take account of the inherent curvature of an ophthalmic lens of this kind, and possible variation in its thickness, in particular when it is a so-called progressive lens the focal length of which varies continually, and of the "meniscus" which the ring or surround in which the lens is to be mounted itself features, in other words the inherent curvature of this ring or surround, it is necessary to displace the lens parallel to its axis during its rotation relative to the beveling grinding tool, so that its point of contact with the latter follows an appropriate trajectory between its circumferential edges.
In other words, it is necessary to provide for relative axial displacement between the two support shafts concerned, that carrying the ophthalmic lens and that carrying the beveling grinding tool.
The relative axial displacement which thus has to be applied to one of these support shafts, hereinafter referred to for convenience as the mobile support shaft, may be provided manually.
This presupposes a certain degree of dexterity on the part of the operator, the corresponding beveling of the ophthalmic lens being done merely by eye.
Consequently, the result is always to some extent approximate.
Alternatively, relative axial displacement of the mobile support shaft may be freely effected by the utilization of a beveling grinding tool with two slant edges into the groove in which the entire edge of the lens is inserted so that it is automatically and continuously centered.
Although this arrangement is advantageously very simple to implement, in practice it is suitable only for relatively thin and uniformly curved ophthalmic lenses.
When it is a matter of processing a thick edged ophthalmic lens, especially a toriodal ophthalmic lens, the necessary width of a beveling grinding tool with two slant edges rapidly becomes unacceptable.
As a corollary to this, when it is a matter of processing a progressive ophthalmic lens, for example, and thus one which has a thickness varying along its circumference, the bevel to be formed may "overstep" the bounds of this circumference in the thinnest part of the lens concerned or, in other words, go wrong in this area.
It is for this reason that it has been proposed, in constructing grinding machines for processing this type of lens in particular, to control or in other words guide the relative axial displacement of the two support shafts, by means of an appropriate control system, so that the point of contact of the ophthalmic lens with the beveling grinding tool follows a predetermined trajectory.
Various methods of control may be envisaged.
The grinding machine described in French patent application No. 83 16574 filed Oct. 18, 1983, for which the present invention is particularly but not necessarily exclusively intended, used one of these methods.
Be this as it may, it is obvious that for a relatively thin ophthalmic lens of uniform curvature such controled guided beveling is unnecessary.
In practice, the practitioner would find it beneficial to have a grinding machine providing for either guided beveling or non-guided beveling.
To provide a grinding machine of this kind is one object of the present invention.